This application is related to U.S. application Ser. No. 08/365,656, filed Dec. 29, 1994 as "Moriarty 3," and U.S. Provisional Application Ser. No. 60/009,353, filed Dec. 29, 1995 as "Moriarty 5" and converted to U.S. application Ser. No. 08/769,984 on Dec. 19, 1996, all three of which are incorporated herein by reference.
1. Field of the Invention
The invention relates generally to power converters, and, more particularly, to a power converter with a boost circuit for powering a load from an AC source. The invention is well-suited for resonant ballast applications.
2. Description of the Related Art
The two primary functions of a fluorescent lamp ballast are well known. First, to create enough voltage to start the lamp, and second, to limit the current once the lamp is started. For many years, both functions of a fluorescent lamp ballast were achieved by the use of magnetics, with a transformer to step up the voltage and an inductor to limit the current.
In recent years, electronic ballasts have been frequently used instead of magnetic ballasts to control fluorescent lamps and other non-linear loads. Compared to magnetic ballasts, electronic ballasts are smaller and lighter, do not have flicker associated with 60-Hz power mains, and are more efficient. Electronic ballasts are especially desirable for powering the compact, consumer fluorescent lamps that are becoming more popular.
Most compact electronic ballasts consist of discrete components, including power transistors and pulse transformers. A typical electronic ballast includes a half-bridge totem pole driver formed from two discrete power MOSFETs of bipolar transistors. The half-bridge output drives a resonant load by means of feedback from a pulse transformer whose primary winding is connected in series with the load. The two secondary windings of the transformer are connected to the inputs of the two half-bridge transistors such that the load is driven synchronously. The LC resonating elements provide substantially sinusoidally varying voltage and/or current waveforms. Driving the load in such a self-synchronous fashion allows a switching frequency at the load that is much higher than the line frequency. As a result, much smaller reactive components may be used to reduce the bulk and size of the ballast.
A disadvantage of typical electronic ballasts is a low power factor. That is, the current drawn from the power source is out of phase with the voltage of the source. Typically, the current leads the voltage, e.g., by approximately 50 to 60%. The low power factor is largely due to a peak detector rectifier that allows power to be drawn from the source only when the source voltage is higher than the load voltage. The low power factor not only wastes energy, but tends to inject high-frequency harmonics into the line. The total harmonic distortion might violate FCC regulations and/or disrupt other circuits connected to the line. A large capacitor in shunt with the line reduces total harmonic distortion, but is also likely to reduce the power factor.
A buck-boost power converter that operates with a high power factor is disclosed in U.S. application Ser. No. 08/365,656. A similar buck-boost power converter is disclosed in J. Moriarty et al., "New Integrated Electronic Ballast Chip Set," Proceedings of the Twenty-Ninth International Power Conversion Conference, September 1994, pp. 280-287, which is incorporated herein by reference. These buck-boost power converters are simple to control and have a high power factor since current drawn from the source to charge an inductor is in phase with the voltage of the source. A disadvantage, however, is that high peak currents result in high power dissipation and harmonic distortion.
Therefore, a need exists for a power converter that operates with a high power factor and low harmonic distortion.